(1) Field of the Invention
The present invention relates to the encoding means of a digital data communication transmitter.
(2) Prior Art
With respect to the prior art, encoding is the process by which digital data bits or analog signal information is transmitted over a communication channel. The use of code redundancy will increase the information certainty of the receiving system. Codes have most often been employed to satisfy the demands of the communication system. Codes are used to a great advantage in spread spectrum systems. A spread spectrum system is one that employs a great deal more bandwidth in transmitting its information than would normally be required. Error detecting and correcting codes for instance are often employed in spread spectrum systems on the data channel to protect the information from interference induced errors. Performing a correlation process in a spread spectrum receiver acts to overcome interference and thus to improve the decoder performance. Spread spectrum systems of the direct sequence type are very similar to pulse code modulation (PCM) systems except the usual PCM codes are replaced by very long code sequences. There are various PCM codes which are described for example in "Digital Computer Technology and Design" Vol. II, Pages 12.26-12.27 by W. H. Ware, John Wiley and Sons, 1963.
A number of encoding schemes can be used for transmission. The desirable features of the code employed are as follows:
1. It should be possible to derive timing information from the encoded waveform.
2. The process of decoding or demodulation should result in a carrier detect signal.
3. The signal element should have state changes to prevent repetitions of the same code and information states (i.e., strings) from producing low frequency signal spectrum components.
4. The average value of the signal should be zero to facilitate A.C. coupling.
5. The total power transmitted for an encoded "zero" and "one" should be equal so that the transmitted energy is independent of string sequence form.
6. If the encoded signal is waveform integrated to reduce high frequency spectral components then conditions 4 and 5 must be satisfied.
The above conditions place strict constraits on the encoder design. If a wideband data channel is available on the communication medium, then spread spectrum techniques can be employed. The use of longer code sequences, than the usual pulse code modulation (PCM) codes, can be used effectively to produce an encoded signal to meet the above constraits. If a plurality of transmitters are to share a common communication channel, then the detection of multiple transmission of similar spectrum signals is important. This places another constrait on the encoder. The out-of-phase correlation properties of the encoded signal are important. The correlation of "zero" and "one" canonical code sequences should produce a collision detectable signal. Also the correlation of multiple signals formed by any combination of "zero" and "one" canonical code sequences should be detectable. Nevertheless, the code sequences generated by the encoder should be able to protect the information from noise interference induced errors. These are some of the problems this invention overcomes.